When reseller creates a new subscription, the owner of the created subscription gets an email notification. Notification is being sent from administrator email address by default.
It is expected that the above message would be sent from the reseller's email address.
The required functionality - sending email notifications to customers which belong to a reseller from corresponding reseller email address, was not implemented yet. The feature request number 1775966 regarding the matter exists.
Feature will be implemented in future releases.
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There is a possibility to change general email address which would be used for send notifications to all customers and resellers. You can read more information about this in Parallels Plesk Automation Operation Guide, page 96, Setting Up Notification Messages.